The present invention relates to a forming drum for building a tire, and more particularly, to a tire belt forming drum for forming a strip-like belt member having a steel cord buried therein into an annular shape.
Generally, a belt forming drum has such structure that a diameter of an outer circumferential surface of a cylinder onto which a strip-like belt member is to be stuck can be arbitrarily varied so that a formed annular belt of various sizes may be obtained. More particularly, as shown in FIG. 4, a belt forming drum is composed of a plurality of segments 01 severed in the circumferential direction, and as a result of movement of these segments in the radial direction, the outer diameter of the cylinder expands or collapses. In order to cover the gaps between the segments upon expansion, heretofore an annular rubber band 02 was provided so as to surround the outer circumference of the cylinder.
However, the rubber band 02 has a limit in elongation, and so, with only one kind of rubber band, the size of the annular belt that can be formed is extremely limited. Therefore, the scope of the applicable forming size was enlarged by preliminarily preparing a several kinds of rubber bands having different circumferential lengths and making use of them selectively according to necessity.
But, such type of rubber bands are manufactured to be relatively thin in order to increase an expanding/collapsing proportion, hence the work of replacing the rubber band is very difficult, and especially the rubber band replacement work on a belt forming drum for forming a large-sized tire was a laborious time-consuming work. With regard to a sticking position of a belt member onto a drum also, since there is no way for positively fixing a belt member to segments, there was inconvenience that a sticking position of a belt member was displaced in the course of sticking or an annular belt was disengaged from a drum due to rotation of the drum after sticking.
Since a rubber band has a tendency of gradually elongating while it is used, there was also an inconvenience that when a forming drum is collapsed in diameter and a formed annular belt is withdrawn therefrom, a rubber band elongated longer than the circumferential length of the forming drum when it has the minimum diameter, would be withdrawn together with the annular belt. Furthermore, in the event that a rubber band should be damaged, there was a fear that the rubber might be suddenly broken during expansion of a drum.
Since the forming drum provided with a rubber band surrounding the outside of a plurality of segments is accompanied by the above-mentioned shortcomings, a forming drum not making use of a rubber band was proposed in U.S. Pat. No. 3,932,256. FIGS. 5a, 5b, 5c, are partial cross-section views respectively showing an expanded (the maximum diameter) condition E, an intermediate condition I and a collapsed (the minimum diameter) condition C of the proposed forming drum, wherein reference numerals 01a and 01b indicate two adjacent segments. Between the segments 01a and 01b is provided a cover plate 02a slightly bent at its center, and a part of the cover plate 02a on the side of the segment 01a is fixedly secured to the outer surface of the segment 01a by means of a flat head screw 03. The other end of the cover plate 02a is overlapped on the segment 01b. To the segment 01b is also fixedly secured a similar cover plate 02b.
The respective segments are movable in the radial direction, and under the collapsed condition illustrated in FIG. 5c, the segments come so close to one another that they nearly come into contact with one another to reduce the outer diameter of the drum. At this time, the cover plate 02a would deeply overlap on the surface of the segment 01b to form a nearly cylindrical outer circumferential surface.
If the segments are moved radially outwards from this collapsed condition, then they take an intermediate condition shown in FIG. 5b. Under this condition, although the segments 1a and 1b are separated from one another, the cover plates 02a cover the gap spaces therebetween and would form an almost smooth cylindrical outer circumferential surface.
If the segments are further moved radially outwards from this intermediate condition, then the expanded condition shown in FIG. 5a is realized, where an also smooth cylindrical outer circumferential surface having the maximum outer diameter can be formed. In this way, the outer diameter of the drum can be arbitrarily varied from the condition where the segments are placed close to one another, up to the external condition where the free end portions of the cover plates can overlap on the outer surfaces of the adjacent segments, and in the course of the variation a nearly smooth cylindrical outer circumferential surface can be formed.
A strip-like belt is wound around the thus formed cylindrical outer circumferential surface, stuck there and brought into tight contact with the drum, and thereby an annular belt having a desired size can be formed. However, there is no means for positively bringing the strip-like belt into tight contact with the drum, but only the stickiness of rubber forming the strip-like belt is relied upon. Accordingly, when the strip-like belt is wound around the forming drum, an inconvenience that a small gap space may be produced between the cylindrical outer circumferential surface of the drum and the belt member and hence the opposite ends of the belt member upon finishing of the winding may not coincide with each other, resulting in a gap space therebetween, is liable to occur.